Charge balance modeling system for MRI sequences

ABSTRACT

An imaging system comprises determination of a charge block for each building block of an MRI pulse sequence and for each readout event of the MRI pulse sequence, determination, for each charge block, of a charge per request associated with the charge block, determination, for each charge block, of an associated charge reduction based on a charge per request associated with the charge block and on a charge available to the charge block after execution of a previous charge block of the MRI pulse sequence, determination, for each charge block associated with a non-zero charge reduction, of a flip angle of a corresponding building block of the MRI pulse sequence based on a charge per request and a charge reduction associated with the charge block, and control of a radio frequency system to deliver the MRI pulse sequence based on the determined flip angles of each building block of the MRI pulse sequence corresponding to a charge block associated with a non-zero charge reduction.

BACKGROUND

An MRI scanner generates images of patient anatomy based on sequences ofRF pulses. An RF power amplifier generates and stores the electricalcharge needed to emit the pulses. Currently, reliable image quality isachieved by sizing the RF power amplifier and other RF transmissionparts based on the most charge-intensive foreseen imaging applications.For example, the RF components of an MRI scanner are typically selectedsuch that they may generate and store enough charge to successivelyexecute two energy-demanding pulse sequence blocks (i.e., an adiabaticT2-preparation module and a single-shot TrueFisp readout) withoutrunning out of charge. However, these pulse sequence blocks are used inan imaging application which represents approximately 1% of all MRIscans, and therefore the total energy capacity of the RF components isusually underutilized. Even so, in the case of some patients, this totalenergy capacity is still insufficient for executing the above-mentionedenergy-demanding pulse sequence blocks.

It is desirable to produce improved MRI image quality for a given chargegeneration and storage capability. Such improvements may allow the useof less expensive RF components than conventional MRI scanners, whilemaintaining suitable image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI system according to someembodiments.

FIG. 2 illustrates charge blocks of a pulse sequence according to someembodiments.

FIGS. 3A and 3B comprise a flow diagram of a process according to someembodiments.

FIG. 4 is a tabular representation of charge block parameters accordingto some embodiments.

FIG. 5 is a tabular representation of charge block parameters accordingto some embodiments.

FIG. 6 is a tabular representation of charge block parameters accordingto some embodiments.

FIG. 7 is a tabular representation of charge block parameters accordingto some embodiments.

FIG. 8 is an outward view of a user interface for illustrating a changedflip angle associated with a charge block according to some embodiments.

FIG. 9 is an outward view of a user interface for illustrating a changedflip angle associated with a charge block according to some embodiments.

DETAILED DESCRIPTION

The following description is provided to enable any person in the art tomake and use the described embodiments. Various modifications, however,will remain readily apparent to those in the art.

Generally, some embodiments provide charge balance assessment, reductionand redistribution among charge blocks of an MRI pulse sequence. Someembodiments allow for a controlled distribution of an RF poweramplifier's overall available charge to the individual charge blockswhich enforces a charge-compliant protocol by reducing the flip angle ofthe refocusing pulses of the readout train. More specifically, flipangles and implicit time delays may be adjusted so that a) the RF poweramplifier does not run out of energy (i.e., the entire pulse sequencerun completes successfully), and that b) each charge block receivessufficient energy to produce suitable image quality.

For example, some embodiments ensure that a single echo train (i.e.,excitation pulse plus multiple subsequent refocusing pulses) does notexceed the maximum charge available. The charge may be calculated asmaximum charge stored in the RF power amplifier at the beginning of themeasurement, minus the charge used by the excitation pulse and therefocusing pulses, plus the charge provided by the continuous rechargingwhile the echo train runs. The charge created by the continuousrecharging may be modeled as proportional to the duration of the echotrain. In case more charge is required for a given pulse than will beavailable, the refocusing pulses' flip angle is reduced so that zero (ormore) charge remains at the end of the first echo train. Additionally,some embodiments determine whether the charge is also sufficient forexecuting the remaining effective repetitions of the scan. If not, afurther flip angle reduction is applied so that zero (or more) chargeremains at the end of the last echo train.

FIG. 1 illustrates MRI system 1 according to some embodiments. MRIsystem 1 includes MRI chassis 2, which defines bore 3 in which patient 4is disposed. MRI chassis 2 includes polarizing main magnet 5, gradientcoils 6 and RF coil 7 arranged about bore 3. According to someembodiments, polarizing main magnet 5 generates a uniform main magneticfield (B₀) and RF coil 7 emits an excitation field (B₁).

According to MRI techniques, a substance (e.g., human tissue) issubjected to a main polarizing magnetic field (i.e., B₀), causing theindividual magnetic moments of the nuclear spins in the substance toprocess about the polarizing field in random order at theircharacteristic Larmor frequency, in an attempt to align with the field.A net magnetic moment M_(z) is produced in the direction of thepolarizing field, and the randomly-oriented magnetic components in theperpendicular plane (the x-y plane) cancel out one another.

The substance is then subjected to an excitation field (i.e., B₁)created by emission of a radiofrequency (RF) pulse, which is in the x-yplane and near the Larmor frequency, causing the net aligned magneticmoment M_(z) to rotate into the x-y plane so as to produce a nettransverse magnetic moment M_(t), which is rotating, or spinning, in thex-y plane at the Larmor frequency. The excitation field is terminatedand signals are emitted by the excited spins as they return to theirpre-excitation field state. The emitted signals are detected, digitizedand processed to reconstruct an image using one of many well-known MRIreconstruction techniques.

An RF pulse may be emitted as a magnetization preparation step in orderto enhance or suppress signals from certain tissue so as to generatedesired levels of contrast in the resulting image. For example, aninversion, or saturation, pulse is used in non-contrast-enhancedangiography to suppress venous blood in order to highlight the arterialsystem.

Gradient coils 6 produce magnetic field gradients G_(x), G_(y), andG_(z) which are used for position-encoding NMR signals. The magneticfield gradients G_(x), G_(y), and G_(z) distort the main magnetic fieldin a predictable way so that the Larmor frequency of nuclei within themain magnetic field varies as a function of position. Accordingly, anexcitation field B₁ which is near a particular Larmor frequency will tipthe net aligned moment M_(z) of those nuclei located at field positionswhich correspond to the particular Larmor frequency, and signals will beemitted only by those nuclei after the excitation field B₁ isterminated.

Gradient coils 6 may consist of three windings, for example, each ofwhich is supplied with current by an amplifier 8 a-8 c in order togenerate a linear gradient field in its respective Cartesian direction(i.e., x, y, or z). Each amplifier 8 a-8 c includes a digital-analogconverter 9 a-9 c which is controlled by a sequence controller 10 togenerate desired gradient pulses at proper times.

Sequence controller 10 also controls the generation of RF pulses by RFsystem 11 and RF power amplifier 12. RF system 11 and RF power amplifier12 are responsive to a scan prescription and direction from sequencecontroller 10 to produce RF pulses of the desired frequency, phase, andpulse amplitude waveform. The generated RF pulses may be applied to thewhole of RF coil 7 or to one or more local coils or coil arrays. RF coil7 converts the RF pulses emitted by RF power amplifier 12, viamultiplexer 13, into a magnetic alternating field in order to excite thenuclei and align the nuclear spins of the object to be examined or theregion of the object to be examined. As mentioned above, RF pulses maybe emitted in a magnetization preparation step in order to enhance orsuppress certain signals.

The RF pulses are represented digitally as complex numbers. Sequencecontroller 10 supplies these numbers in real and imaginary parts todigital-analog converters 14 a-14 b in RF system 11 to createcorresponding analog pulse sequences. Transmission channel 15 modulatesthe pulse sequences with a radio-frequency carrier signal having a basefrequency corresponding to the resonance frequency of the nuclear spinsin the volume to be imaged.

RF coil 7 both emits radio-frequency pulses as described above and scansthe alternating field which is produced as a result of processingnuclear spins, i.e. the nuclear spin echo signals. The received signalsare received by multiplexer 13, amplified by RF amplifier 16 anddemodulated in receiving channel 17 of RF system 11 in a phase-sensitivemanner. Analog-digital converters 18 a and 18 b convert the demodulatedsignals into a real part and an imaginary part.

Computing system 20 receives the real and imaginary parts andreconstructs an image therefrom according to known techniques. System 20may comprise any general-purpose or dedicated computing system.Accordingly, system 20 includes one or more processing units 21 (e.g.,processors, processor cores, execution threads, etc.) configured toexecute processor-executable program code to cause system 20 to operateas described herein, and storage device 22 for storing the program code.Storage device 22 may comprise one or more fixed disks, solid-staterandom access memory, and/or removable media (e.g., a thumb drive)mounted in a corresponding interface (e.g., a USB port).

Storage device 22 stores program code of control program 23. One or moreprocessing units 21 may execute control program 23 to cause system 20 toperform any one or more of the processes described herein. For example,one or more processing units 21 may execute control program 23 to causesystem 20 to change parameters of charge blocks of a pulse sequencebased on available charge as described herein. Pulse sequences 26include data specifying the parameters of pulse sequences and theirconstituent building blocks and readout events. Pulse sequences 26 mayinclude pulse sequences which have been input to the process describedherein as well as pulse sequences which have been changed as a resultthereof.

One or more processing units 21 may execute control program 23 to causesystem 20 to receive the real and imaginary parts of a received RFsignal via MR system interface 24 and reconstruct an image therefromaccording to known techniques. Such an image may be stored amongacquired images 28 of storage device 22.

One or more processing units 21 may also execute control program 23 toprovide instructions to sequence controller 10 via MR system interface24. For example, sequence controller 10 may be instructed to initiate adesired pulse sequence of pulse sequences 26. In particular, sequencecontroller 10 may be instructed to control the switching of magneticfield gradients via amplifiers 8 a-8 c at appropriate times, thetransmission of radio-frequency pulses having a specified phase andamplitude at specified times via RF system 11 and RF amplifier 12, andthe readout of the resulting magnetic resonance signals.

Acquired images 27 may be provided to terminal 30 via terminal interface25 of system 20. Terminal interface 25 may also receive input fromterminal 30, which may be used to provide commands to control program 23in order to control sequence controller 10 and/or other elements ofsystem 1. The commands may specify pulse sequence parameter values whichare used by system 20. Such parameter values may include original andchanged flip angles associated with a pulse sequence building block.Terminal 30 may simply comprise a display device and an input devicecoupled to system 20. In some embodiments, terminal 30 is a separatecomputing device such as, but not limited to, a desktop computer, alaptop computer, a tablet computer, and a smartphone.

Each element of system 1 may include other elements which are necessaryfor the operation thereof, as well as additional elements for providingfunctions other than those described herein. Storage device 22 may alsostore data and other program code for providing additional functionalityand/or which are necessary for operation of system 20, such as devicedrivers, operating system files, etc.

Pulse sequences, such as pulse sequences 26, are commonly represented asa series of building blocks which are stored in the order in which theywill be executed. FIG. 2 illustrates charge blocks of pulse sequence 200according to some embodiments. One charge block is assigned to eachbuilding block of pulse sequence 200 and to each series of readoutevents.

Pulse sequence 200 combines a high-flip angle MT-pulse series (i.e., 10pulses) with a trailing IR pulse, a time delay, and a single shotTrueFisp readout with short echo-spacing. According to the illustratedprotocol, the effective repetition includes two heartbeats depictedusing dashed lines, respectively. In the first heartbeat, the blocks area time delay after the R-wave ([00] FillPre), the preparation consistingof 10 MT pulses ([01] MTPrep), the IR preparation ([02] IRPrep), anothertime delay to realize the protocol's TI ([03] FillToRO), the ramp-up ofthe Trufi readout ([04] PreROT), and the readout train ([05] ROTrain).In the second heartbeat, the reference data is acquired by executing thesame timing, but replacing the preparations with time delays andchoosing low flip angles for the reference readout ([11] RefROT) and itsramp-up ([10] REFPre).

FIGS. 3A and 3B comprise a flowchart of process 300 according to someembodiments. In some embodiments, various hardware elements of system 1(e.g., one or more processors) execute program code to perform process300. Process 300 and all other processes mentioned herein may beembodied in processor-executable program code read from one or more ofnon-transitory computer-readable media, such as a floppy disk, adisk-based or solid-state hard drive, CD-ROM, a DVD-ROM, a Flash drive,and a magnetic tape, and then stored in a compressed, uncompiled and/orencrypted format. In some embodiments, hard-wired circuitry may be usedin place of, or in combination with, program code for implementation ofprocesses according to some embodiments. Embodiments are therefore notlimited to any specific combination of hardware and software.

Initially, at S305, a charge block is determined for each building blockof a pulse sequence of an MRI scan and for each readout event of thepulse sequence. The pulse sequence is an effective repetition accordingto some embodiments. Several properties are determined for each chargeblock, many of which may be determined from the building blocks whichconventionally define the pulse sequence. One such property is a pointerto the sequence building block represented by the charge block. Thispointer may be used to pass information to the corresponding sequencebuilding block, for example to instruct the sequence building block toreduce its flip angle.

According to some embodiments, the properties of a charge block furtherinclude a charge per request and a duration. The duration of the chargeblock equals the duration per request for individual pulse sequencebuilding blocks. For readout events, the duration of the correspondingcharge block equals the duration per request of a single readout eventmultiplied by the number of consecutive readout events, which are inturn grouped together into one charge block.

Some conventional MRI systems allow a user to query a charge per requestof a pulse sequence building block. Such systems compute the charge perrequest based on the specified properties of the building block and onsystem/site-specific parameters. These system/site-specific parametersmay depend on the cabling, coils, dampening and other electricalparameters of the MRI system. Examples of determination of a charge perrequest will be provided below, so as not to interrupt the presentdescription of process 300.

Other properties associated with each charge block at S305 may include aflag (e.g., “bLastBlockInTReff”), which indicates a block which is thelast block within the effective repetition, and a flag indicatingwhether a charge reduction is allowed (e.g., “bScalingAllowed”). Thislatter flag may be a constant property of a building block and may beset to false, for example, for charge blocks associated with adiabaticpulses. A flag (e.g., “bFairExchange”) may also indicate whether thecharge block will take part in the fair exchange of charge as describedbelow. This flag may initially be set to false.

Still other properties may be initialized to zero for each charge blockat S305. These properties may be used in process 300 and may include arequired charge reduction, a fair exchange charge reduction, and anadditional charge reduction for multiple effective repetitions. Thesignificance and usage of these properties will be evident from theforegoing description.

Each charge block may further be associated with an importance factor,which represents the importance of maintaining the originally-determinedcharge per request for the block, relative to the importance factors ofthe other charge blocks. An optional text string may also be determinedto describe the function of the charge block.

FIG. 4 is a tabular representation of a subset of the above-describedproperties determined at S305 for each of the charge blocks of pulsesequence 200. The properties are listed in the following order: index,description, original charge, block duration, and bLastInTReff.According to some embodiments, process 300 is implemented in thesequence framework so it is available to all pulse sequences andlabor-intensive separate implementation into each sequence would beunnecessary. The table of FIG. 4 could be implemented as linked list orchain, and its entries (i.e., the charge block properties) could beimplemented into the basic building block class so as to then beavailable to all derived building block classes.

The block duration is given in microseconds in FIG. 4. The standard unitof charge is Coulomb (i.e., Ampere seconds). However, according to thecharge calculation provided by some systems, the unit of charge ismicroseconds. Converting from the standard unit of charge requires thereference voltage, the cable dampening between the RF power amplifierand the excitation coil, and other system parameters.

At S310, a charge reduction is determined for each charge block based onthe charge available to the charge block and the charge required by thecharge block. FIG. 5 illustrates a table of the FIG. 4 charge blocks andsome associated properties for the purpose of explaining S310 accordingto some embodiments.

The column “original charge” indicates the net charge per requestassociated with the charge block, and the “charge sum” indicates theamount of charge remaining after execution of the charge block. It willbe assumed that the capacitors of the RF power amplifier arecontinuously recharging such that some blocks (e.g., [00] FillPre) areassociated with a positive original charge (i.e., the available chargeis greater at the end of the charge block than at the beginning of thecharge block).

A charge block that requires a larger “original charge” than the current“charge sum” would cause the RF power amplifier to run out of charge. Toavoid this, the block is assigned a “required charge reduction” at S310.The block is therefore allowed to use its original charge per requestminus the required charge reduction. Practically, this may achieved byreducing the flip angle of the block.

The present example assumes that the maximum charge that can be storedis Q_max=+20000.00. Assuming that the sequence begins at full charge,the available charge at the end of [00] FillPre will still be +20000.00even though [00] FillPre is associated with a positive original chargeof +16,707. The next charge block [01] MTPrep is associated with anegative charge of −18964.87, therefore, as shown in the associated rowof the charge sum column, the available charge at the end of chargeblock [01] MTPrep is +20000.00+(−18964.87)=+01035.13.

[02] IRprep requires more charge (i.e., −01586.20) than is availableafter [01] MTPrep (i.e., +01035.13), therefore [02] IRprep is assigned arequired reduction equal to the shortfall in charge (i.e.,−01586.20+(+01035.13)=−00551.07). The charge sum of [02] IRPreptherefore equals the charge sum of [01] MTPrep plus the original chargeof [02] IRprep—the required reserve of [02] IRprep, which equals zero.

[03] FillToRO and [04] PreROT are both associated with a positive chargebalance (i.e., they create more charge than they use) so that +00567.42of stored charge is available after [04] PreROT. [05] ROTrain requires−15113.13 of charge, which is not met by the +00567.42 of availablestored charge. Accordingly, [05] ROTrain is assigned a requiredreduction of −15113.13+(+00567.42)=−14545.71, as shown in FIG. 5. Themagnitude of this required reduction would require considerable scalingdown of its flip angle. The below-described “fair exchange” mechanismmay allow for a smaller (in absolute value) required reduction andtherefore a larger flip angle.

All remaining blocks of the sequence except for [11] RefROT have a netcharge generation effect so that the charge sum is increasing. No chargereduction is required for [11] RefROT because its original charge perrequest (i.e., −01343.39) is met by the available charge (i.e.,+20000.00).

Next, at S315, a set of charge blocks is determined which are not to beassociated with a charge reduction. Such charge blocks may be thoseblocks for which a lesser charge would render them inoperable or causesignificant deterioration in image quality, and may be identified by the“bScalingAllowed” mentioned above. For example, [02] IRPrep is anadiabatic pulse that should therefore not be scaled down in flip angle.

Accordingly, at S320, for each of the set of blocks determined at S315,a determined charge reduction is assigned to an immediately prior blockin the sequence which is not itself a member of the set of blocksdetermined at S315. Therefore, the required reduction of [02] IRPrep isassigned to the required reduction of [01] MTPrep as shown in FIG. 6.

Consequently after S320, the required reduction of [02] IRPrep is zero,whereas the required reduction of [01] MTPrep equals its former valueplus the former value of [02] IRPrep, resulting in −551.07. In theembodiment shown in the example, the original charge o [02] IRPrep isalso moved to [01] MTPrep, resulting in a new original charge of [02]IRPrep (i.e., zero) and a new original charge of [01] MTPrep (i.e.,−20,551.07=the sum of −551.07 and −18,964.87). As a result, the chargesum of [01] MTPrep is exactly zero. This charge sun results from thespecific bookkeeping of the illustrated embodiment and does not indicatethat no charge is left to execute [02] IRPrep. Rather, the chargerequired for executing [02] IRPrep in unaltered form is alreadyanticipated and accounted for by [01] MTPrep. The depicted embodimentshall be understood as non-limiting and other embodiments are possiblefor which the original charge remains unchanged and an additional chargeblock property reflects the redistributed charge.

Charge blocks are identified for inclusion in an exchange pool at S325.According to some embodiments, the exchange pool charge blocks are thosewhich are associated with a non-zero charge reduction (e.g., [01] MTPrepand [05] ROTrain), and those associated with a large original chargerequest (e.g., >50% of Q_max), even if the latter charge blocks are notassociated with a charge reduction. As shown in FIG. 7, exchange poolcharge blocks are identified by setting the corresponding flag“bPartofShare” to true.

At S330, the total charge reduction associated with the exchange poolcharge blocks is redistributed among the exchange pool charge blocks.According to some embodiments, the amount to which a charge blockcontributes to the redistribution is proportional to the original chargeper request of the charge block. For example, a new charge reduction maybe assigned to an exchange pool charge block by multiplying the totalrequired charge reduction (i.e., the sum of all required chargereductions of all charge blocks) by the ratio of the charge block'soriginal charge to the sum of the original charges of all the exchangepool charge blocks.

The new charge reduction determined for each exchange pool block at S330is equal to the previously-determined required reduction plus a “fairexchange reduction”. For clarity, the required reduction and the fairexchange reduction may be saved in separate table columns as shown inFIG. 7.

In the specific example of FIG. 7, charge blocks [01] MTPrep and [05]ROTrain are flagged as exchange pool charge blocks. The total chargereduction associated with all charge blocks is(−551.07+(−14,545.71))=−15,096.78. In the case of charge block [01]MTPrep, the above-mentioned ratio is−20,551.07/(−20,551.07−15,113.13)=0.5762. Accordingly, in someembodiments, the new charge reduction determined for charge block [01]MTPrep is −15,096.78*0.5762=−8,699.34. The fair exchange reduction forcharge block [01] MTPrep is therefore −8699.34−(−551.07)=−8,148.27.

Since there are only two blocks in the exchange pool, this fair exchangereduction is distributed as additional charge to charge block [05]ROTrain as shown in FIG. 7. The distributed value (i.e., +7,850.49) mayalso be computed as described above. Consequently, [05] ROTrain isrequired to save less charge than originally determined and cantherefore run with a higher flip angle than with theoriginally-determined charge reduction. FIG. 7 also shows updates to thecharge sum column based on the new net charge reductions (i.e., requiredreduction+fair exchange reduction) of charge blocks [01] MTPrep and [05]ROTrain.

For sequences with a short effective repetition and/or many effectiverepetitions, it is possible that the RF power amplifier runs out ofcharge before all effective repetitions have been played out.Accordingly, at S335, it is determined whether a charge deficit existsafter the last effective pulse sequence repetition. For example, FIG. 7shows the charge sum after [11] RefROT (i.e., the total remaining chargeat the end of the last charge block) as +18,656.61. Accordingly, 20,000(charge at start of repetition)−18,656.61 (charge at end ofrepetition)=1,343.39 charge is lost during each effective TR. 20,000 isthe maximum charge the RF power amplifier can store, and the ratio20,000/1,343.39 is 14.88. Therefore, the RF power amplifier would runout of charge during the 15^(th) repetition of the effective TR.

If a charge deficit per repetition exists, for example 1,343.39 ascalculated above, a total additional charge reduction is determinedbased on the charge deficit per repetition and the total number ofrepetitions. In some embodiments, the total additional charge reductionis equal to the determined charge deficit per repetition multiplied bythe total number of repetitions.

A portion of the total additional charge reduction is assigned to eachblock at S345. The portion may be assigned by dividing the totaladditional charge reduction by the number of effective repetitions toobtain the required charge reduction per effective repetition, and thenassigning an additional charge reduction per charge block. For eachcharge block, the latter is calculated by multiplying the requiredcharge reduction per effective repetition by the ratio of the maximumpossible charge saving of this charge block to the sum of the maximumpossible charge savings of all charge blocks within the repetition. Themaximum possible charge saving of a given charge block is equal to theoriginal charge in the block minus the sum of the already applied chargereductions (i.e., maximum possible charge reduction=originalcharge—required reduction—fair exchange reduction).

At S350, the flip angle of each charge block associated with a netcharge reduction is recalculated based on the net charge reduction.Recalculation of the flip angle of a charge block ensures that thecharge block actually saves the charge the determined amount of charge.In the present example, flip angles are recalculated for charge blocks[01] MTPrep and [05] ROTrain based on their net charge reductions (i.e.,required reduction+fair exchange reduction+additional charge reduction).Recalculation of a flip angle may comprise scaling down the flip anglein proportion to the ratio of the original charge of the block to thereduced charge of the block (i.e., new flip angle=original flipangle*original charge/(original charge+required reduction+fair exchangereduction+additional charge reduction)).

According to some embodiments, the flip angles of the preparation andreadout blocks are reduced only on the module level. No protocol flipangles (even if available) are modified. Instead, tooltips may beprovided that show the actually-played angle, and the reduction factorsdue to clipping prevention and due to required charge reserve. Suchtooltips can be provided for the principal parameter “flip angle”, whichis the readout flip angle. Tooltips may also be provided for specificmodules known to consume a lot of charge, such as a T2-preparations.

For preparation modules, the tooltip may be attached to a user interfaceitem logically belonging to the preparation module, for example to themagnetization preparation element. The block's flip angle itself neednot be displayed. One advantage of this tooltip approach over theprotocol modification is independence from patient loading and referencevoltage. A protocol will therefore not become inconsistent if a higherreference voltage is needed for one patient but the protocol was createdwith a lower reference voltage. Another advantage is that the sequencewill run with flip angles as close to the user-desired or predeterminedoptimal flip angles as possible and will use the available chargeaccording to the determined net charge reductions.

FIGS. 8 and 9 show contrast UI 800 and special UI 900, respectively.Tooltips 810 and 910 display the actually-played flip angles calculatedaccording to embodiments of process 300. UI 800 and UI 900 may bepresented to an operator on terminal 30 via execution of control program23).

Also using terminal 30, the operator may issue a command to execute thepulse sequence modified as described herein. Specifically, RF pulses aregenerated based on the parameter values of the modified pulse sequenceas is known in the art (e.g., by RF system 11 and RF amplifier 12 undercontrol of sequence controller 10) and delivered within an MRI imagingsequence.

Embodiments may model different charge storage capacities and rechargerates. Rules could be created that only allow the use of a“Q_max_extreme” once per given period of time and use of a smaller Q_maxotherwise, to prevent overheating of the RF components.

The foregoing provides an example of determination of a charge perrequest according to some embodiments:

Charge (Q) of one readout train in unit ms:

1) Consumed charge:dQUsedReadout_ms=Norm_factor*(dQExcite_ms+num_refoc_pulses*dQRefocusing_ms)=Norm_factor*(−1000.0*getdExciteMagnitudeIntegralVs()−1000.0*num_refoc_pulses*getdRefocusingMagnitudeIntegralVs( ));

2) Regenerated charge:dQRegeneratedDuringReadout_ms=RecoverRate*getIDurationPerRequest_us()/1000.0

3) ChargePerRequest_ms=dQUsedReadout_ms+dQRegeneratedDuringReadout_ms

A negative ChargePerRequest_ms indicates that the associated blockrequires more charge than what is regenerated during itsDurationPerRequest by constant capacitor recharging.

The NormFactor is a function of maximum RF Power amplifier voltage,transmission cable dampening, transmission cable reflection, gainfactors, derating limits, etc. The Norm_Factor may be calculated asfollows:Norm_Factor=dGainVarFactor/dUMax_V/dCableDamping;

where dGainVarFactor is the gain variation factor, which is the ratio ofnominal and real voltage at Tales level (Tales is one of the hardwarecomponents of the scanner), dUMax_V is the maximum transmitter voltagederived from the maximum power that the RF power amplifier can provide,dUMax_V=sqrt(50.0*dRFPAPowerMax), and dCableDamping is the scannersetup-dependent transmit cable dampening which may be obtained byquerying the system properties.

In different scenarios (e.g., depending on certain cable reflectionvalues), further inputs into this equation may be used, for example asdefined below:

// calculate Q-Norm factor if( dDicoReflection <=dDeratingReflectionLimit ) { dQNormFactor = dGainVarFactor / dUMax_V /dCableDamping; } else if( dDicoReflection <=dQuadraticDeratingReflectionLimit ) { dQNormFactor =(1.0+dDicoReflection) / (1.0+dDeratingReflectionLimit) * dGainVarFactor/ dUMax_V / dCableDamping; }  else { dQNormFactor = (1.0 + (dDicoReflection*dDicoReflection)/dQuadraticDeratingReflectionLimit ) /(1.0+dDeratingReflectionLimit) * dGainVarFactor / dUMax_V /dCableDamping;  }

The foregoing diagrams represent logical architectures for describingprocesses according to some embodiments, and actual implementations mayinclude more or different components arranged in other manners. Othertopologies may be used in conjunction with other embodiments. Moreover,each component or device described herein may be implemented by anynumber of devices in communication via any number of other public and/orprivate networks. Two or more of such computing devices may be locatedremote from one another and may communicate with one another via anyknown manner of network(s) and/or a dedicated connection. Each componentor device may comprise any number of hardware and/or software elementssuitable to provide the functions described herein as well as any otherfunctions. For example, any computing device used in an implementationof a system according to some embodiments may include a processor toexecute program code such that the computing device operates asdescribed herein.

All systems and processes discussed herein may be embodied in programcode stored on one or more non-transitory computer-readable media. Suchmedia may include, for example, a floppy disk, a CD-ROM, a DVD-ROM, aFlash drive, magnetic tape, and solid state Random Access Memory (RAM)or Read Only Memory (ROM) storage units. Embodiments are therefore notlimited to any specific combination of hardware and software.

Embodiments described herein are solely for the purpose of illustration.Those in the art will recognize other embodiments may be practiced withmodifications and alterations to that described above.

What is claimed is:
 1. A system comprising: a chassis defining a bore; amain magnet to generate a polarizing magnetic field within the bore; agradient system to apply a gradient magnetic field to the polarizingmagnetic field; a radio frequency system to transmit RF pulses topatient tissue disposed within the bore and to receive signals from thepatient tissue; and a computing system to execute program code to:determine a charge block for each building block of an MRI pulsesequence and for each readout event of the MRI pulse sequence; for eachof the charge blocks, determine a charge per request associated with thecharge block; for each of the charge blocks, determine an associatedcharge reduction based on the charge per request associated with thecharge block and on a charge available to the charge block afterexecution of a previous charge block of the MRI pulse sequence; for eachof the charge blocks associated with a non-zero charge reduction,determine a flip angle of the corresponding building block of the MRIpulse sequence based on the charge per request and the charge reductionassociated with the charge block; and control the radio frequency systemto deliver the MRI pulse sequence based on the determined flip angles ofeach of the building blocks of the MRI pulse sequence corresponding tothe charge block associated with a non-zero charge reduction.
 2. Thesystem according to claim 1, wherein the determination of the associatedcharge reduction based on the charge per request associated with thecharge block further comprises: identification of one or more of thecharge blocks which is associated with a non-zero charge reduction andwhich is not associated with a charge reduction; and for each of theidentified one or more of the charge blocks which is not associated witha charge reduction, moving the associated charge reduction to a priorcharge block which is associated with a non-zero charge reduction. 3.The system according to claim 2, wherein the determination of theassociated charge reduction based on the charge per request associatedwith the charge block further comprises: identify a repetition of two ormore of the charge blocks, each of the repetition of two or more of thecharge blocks being associated with a non-zero charge reduction; andredistribute a total charge reduction associated with each one of therepetition of two or more of the charge blocks among the repetition oftwo or more of the charge blocks based on the charge per requestassociated with each one of the repetition of two or more of the chargeblocks.
 4. The system according to claim 3, wherein the redistributionof the total charge reduction associated with each one of the repetitionof two or more of the charge blocks comprises: for each one of therepetition of two or more of the charge blocks, multiply the totalcharge reduction by the ratio of the charge per request associated withthe one of the repetition of two or more of the charge blocks to the sumof the charges per request of all of the repetition of two or more ofthe charge blocks.
 5. The system according to claim 3, wherein therepetition of two or more of the charge blocks further include ones ofthe charge blocks which are associated with a charge per request greaterthan a threshold amount and which are not associated with a non-zerocharge reduction.
 6. The system according to claim 5, wherein theredistribution of the total charge reduction associated with each one ofthe repetition of two or more of the charge blocks comprises: for eachone of the repetition of two or more of the charge blocks, multiply thetotal charge reduction by the ratio of the charge per request associatedwith the one of the repetition of two or more of the charge blocks tothe sum of the charges per request of all of the repetition of two ormore of the charge blocks.
 7. The system according to claim 1, furthercomprising: a display, wherein the computing system is to executeprogram code to control the display to display a tooltip indicating thedetermined flip angle of one of the building blocks of the MRI pulsesequence.
 8. A computer-implemented method comprising: determining acharge block for each building block of an MRI pulse sequence and foreach readout event of the MRI pulse sequence; for each of the chargeblocks, determining a charge per request associated with the chargeblock; for each of the charge blocks, determining an associated chargereduction based on the charge per request associated with the chargeblock and on a charge available to the charge block after execution of aprevious charge block of the MRI pulse sequence; for each of the chargeblocks associated with a non-zero charge reduction, determining a flipangle of the corresponding building block of the MRI pulse sequencebased on the charge per request and the charge reduction associated withthe charge block; and controlling a radio frequency system to deliverthe MRI pulse sequence based on the determined flip angles of each ofthe building blocks of the MRI pulse sequence corresponding to thecharge block associated with the non-zero charge reduction.
 9. Thecomputer-implemented method according to claim 8, wherein thedetermining the associated charge reduction based on the charge perrequest associated with the charge block further comprises: identifyingone or more of the charge blocks which is associated with a non-zerocharge reduction and which is not associated with a charge reduction;and for each of the identified one or more of the charge blocks which isnot associated with a charge reduction, moving the associated chargereduction to a prior charge block which is associated with a non-zerocharge reduction.
 10. The computer-implemented method according to claim9, wherein the determining the associated charge reduction based on thecharge per request associated with the charge block further comprises:identifying a repetition of two or more of the charge blocks, each ofthe repetition of two or more of the charge blocks being associated witha non-zero charge reduction; and redistributing a total charge reductionassociated with each one of the repetition of two or more of the chargeblocks among the repetition of two or more of the charge blocks based onthe charge per request associated with each one of the repetition of twoor more of the charge blocks.
 11. The computer-implemented methodaccording to claim 10, wherein the redistributing the total chargereduction associated with each one of the repetition of two or more ofthe charge blocks comprises: for each one of the repetition of two ormore of the charge blocks, multiplying the total charge reduction by theratio of the charge per request associated with the one of therepetition of two or more of the charge blocks to the sum of the chargesper request of all of the repetition of two or more of the chargeblocks.
 12. The computer-implemented method according to claim 10,wherein the repetition of two or more of the charge blocks furtherinclude ones of the charge blocks which are associated with a charge perrequest greater than a threshold amount and which are not associatedwith a non-zero charge reduction.
 13. The computer-implemented methodaccording to claim 12, wherein the redistributing the total chargereduction associated with each one of the repetition of two or more ofthe charge blocks comprises: for each one of the repetition of two ormore of the charge blocks, multiplying the total charge reduction by theratio of the charge per request associated with the one of therepetition of two or more of the charge blocks to the sum of the chargesper request of all of the repetition of two or more of the chargeblocks.
 14. The computer-implemented method according to claim 8,further comprising: displaying a tooltip indicating the determined flipangle of each of the building blocks of the MRI pulse sequence.
 15. Anon-transitory computer-readable medium storing program code, theprogram code executable by a computer system to cause the computersystem to: determine a charge block for each building block of an MRIpulse sequence and for each readout event of the MRI pulse sequence; foreach of the charge blocks, determine a charge per request associatedwith the charge block; for each of the charge blocks, determine anassociated charge reduction based on the charge per request associatedwith the charge block and on a charge available to the charge blockafter execution of a previous charge block of the MRI pulse sequence;for each of the charge blocks associated with a non-zero chargereduction, determine a flip angle of the corresponding building block ofthe MRI pulse sequence based on the charge per request and the chargereduction associated with the charge block; and control a radiofrequency system to deliver the MRI pulse sequence based on thedetermined flip angles of each of the building blocks of the MRI pulsesequence corresponding to the charge block associated with the non-zerocharge reduction.
 16. The non-transitory computer-readable mediumaccording to claim 15, wherein the determination of the associatedcharge reduction based on the charge per request associated with thecharge block further comprises: identification of one or more of thecharge blocks which is associated with a non-zero charge reduction andwhich is not associated with a charge reduction; and for each of theidentified one or more of the charge blocks which is not associated witha charge reduction, moving the associated charge reduction to a priorcharge block which is associated with a non-zero charge reduction. 17.The non-transitory computer-readable medium according to claim 16,wherein the determination of the associated charge reduction based onthe charge per request associated with the charge block furthercomprises: identification of a repetition of two or more of the chargeblocks, each of the repetition of two or more of the charge blocks beingassociated with a non-zero charge reduction; and redistribution of atotal charge reduction associated with each one of the repetition of twoor more of the charge blocks among the repetition of two or more of thecharge blocks based on the charge per request associated with each oneof the repetition of two or more of the charge blocks.
 18. Thenon-transitory computer-readable medium according to claim 17, whereinthe redistribution of the total charge reduction associated with eachone of the repetition of two or more of the charge blocks comprises: foreach one of the repetition of two or more of the charge blocks,multiplication of the total charge reduction by the ratio of the chargeper request associated with the one of the repetition of two or more ofthe charge blocks to the sum of the charges per request of all of therepetition of two or more of the charge blocks.
 19. The non-transitorycomputer-readable medium according to claim 17, wherein the repetitionof two or more of the charge blocks further include ones of the chargeblocks which are associated with a charge per request greater than athreshold amount and which are not associated with a non-zero chargereduction.
 20. The non-transitory computer-readable medium according toclaim 19, wherein the redistribution of the total charge reductionassociated with each one of the repetition of two or more of the chargeblocks comprises: for each one of the repetition of two or more of thecharge blocks, multiplication of the total charge reduction by the ratioof the charge per request associated with the one of the repetition oftwo or more of the charge blocks to the sum of the charges per requestof all of the repetition of two or more of the charge blocks.